The operation of electrochemical cells based on gas diffusion electrodes, for example for use as oxygen-consuming cathode in alkali halogenide electrolysis, is basically known and described for example in U.S. Pat. No. 4,657,651.
A gas diffusion electrode is an open-pored structure between electrolyte and gas chamber which has an electrically conductive coating with a catalyst and which is to allow an electrochemical reaction, for example the reduction of oxygen, to take place at the three-phase boundary of electrolyte catalyst and reactant gas in the electrode structure. The boundary layer is generally held in the structure by the surface tension of the electrolyte on the hydrophobic electrode material against the hydrostatic pressure of the electrolyte on the electrode structure. However, only a small pressure drop is permitted between the gas side and the liquid side of the structure which acts as a diaphragm. If the gas-side pressure is too high, the gas finally breaks through the diaphragm, the electrode is disturbed in its function in this region and the electrolytic process is interrupted. If on the other hand the liquid pressure is too high, the three-phase boundary is displaced from the region of the diaphragm containing the catalyst which likewise disturbs the function of the cathode and in the event of a further increase in pressure leads to a liquid breakthrough of electrolyte into the gas chamber. In the case of a vertical electrode arrangement, as required for example in diaphragm electrolysis to permit a favourable discharge of the target product chlorine, this leads to a limitation of the overall height of the gas diffusion electrodes as otherwise at the top gas penetrates into the cathode chamber of the electrode and at the bottom electrolytic liquid penetrates into the gas chamber. The technologically attainable overall height therefore remains limited to approximately 20-30 cm, which is unattractive for commercial diaphragm electrolyzers.
To overcome the problem of pressure compensation, various arrangements have been proposed in the prior art.
In accordance with U.S. Pat. No. 4,657,651, a pressure compensation between the gas chamber and the electrolyte chamber on both sides of a gas diffusion cathode is achieved in that the cathode is divided into individual horizontal chambers which are individually supplied with gas, the gas pressure being regulated by in each case plunging the outgoing gas flow through vertical chambers, in that the depth of said vertical chambers corresponds to the level of the electrolyte in the respective chamber. A disadvantage consists in the high outlay in terms of apparatus, which impedes commercial realization. The pressure in each individual gas chamber must in fact be separately adjusted via respective valves.
German Patent DE 4 444 114 C2 describes an electrochemical half-cell with a gas diffusion electrode wherein pressure compensation between the gas chamber and the electrolyte chamber on both sides of a gas diffusion electrode is achieved in that the gas chamber is divided into two or more gas pockets disposed in cascade fashion one above another, said gas pockets being separated from one another and being open at the bottom towards the electrolyte so that via the opening to the electrolyte the pressure in each gas pocket is in equilibrium with the pressure of the liquid column of electrolyte in the corresponding part of the electrode chamber arranged upstream of the gas diffusion electrode, and wherein a possible gas supply and discharge takes place via the openings to the electrolyte.
However, the known electrolytic cell constructions have a series of technical disadvantages.
In particular in the case of electrolytic cells having a relatively large overall height, it is necessary to prevent the hydrostatic pressure on the lower part of the gas diffusion electrode. Another disadvantage of the half-cell construction according to document DE 4 444 144 is that gas bubbles possibly entrained with the electrolyte accumulate in the region of the electrolyte gap upstream of the gas diffusion electrode during the operation of the cell and can disturb the operation of the cell.
It was also necessary to avoid the disadvantage of being unable to regulate the gas pressure independently of the initially defined structural properties of the cell, which exists for example in the case of DE 4 444 144. Rather, the gas pressure should also be able to be adjusted during operation independently of the electrolyte pressure and optionally changed in relation to an electrolyte pressure which is also independent over the height of the cell.